The Air Force is looking at future types of precision, navigation and timing (PNT) technologies in case its access to its Global Positioning System (GPS) signal is denied or the system is compromised, the service’s top officer said Wednesday.
“I’m pretty confident about precision navigation in the future, but we have to have a varied menu of things to choose from if we want to guarantee the ability to use (GPS),” Air Force Chief of Staff Gen. Mark Welsh told an audience at the National Press Club in downtown Washington. “We have to be able to navigate precisely and set timing precisely to operate the way we are operating as a United States military around the world.”
GPS, consisting of at least 24 satellites 95 percent of the time, is the United States’ PNT system. The latest GPS satellite, GPS IIF-5, was launched in February. The Air Force’s next generation GPS spacecraft, GPS III, is in production, with the Air Force under contract for the first eight satellites. The Air Force awarded GPS III prime contractor Lockheed Martin [LMT] nearly $246 million in March to procure GPS III space vehicles seven and eight (Defense Daily, March 31). Lockheed Martin spokesman Chip Eschenfelder said Wednesday the GPS III-1 is scheduled for launch in 2016.
Multiple media outlets reported Russia’s PNT system, known as Global Navigation Satellite System (GLONASS), went out for about 12 hours in early April. Welsh said one of the “great things” about GPS was its redundant capability, which generally means duplication of technology with intention of increasing reliability.
Dana Goward, president of the Resilient Navigation and Timing (RNT) Foundation, which promotes alternatives to GPS, said Wednesday one of these future PNT technologies is chip-scale atomic clocks, which are highly miniaturized inertial navigation systems that would be able to maintain precise timing and navigation in GPS-denied environments. Chip-scale atomic clocks, first developed by the Defense Advanced Research Projects Agency (DARPA) and the National Institute for Standards and Technology (NIST) in 2004, are smaller than traditional atomic clocks by a factor of 100, down to about the size of a computer chip, and more power-efficient by a factor of 10.
DARPA sent chip-scale atomic clocks into space for the first time in 2011 to be tested aboard the International Space Station (ISS). The chips were inserted into bowling-ball sized satellites on the ISS. Once validated as operational, the satellites were to perform a synchronized maneuver through the ISS cabin. After the experiment, the chips containing the chip-scale atomic clocks were to be removed and tested against the atomic clock onboard the ISS. DARPA calls atomic clocks the most accurate frequency standard and timing devices in the world.
Goward said another potential PNT technology are inertial navigation systems, which he said contain a very precise gyro and accelerometer that tells the system where on earth it started. Calculating off the speed of its own movements and direction, Goward said the accelerometer would know which direction it is going and where it was at any given time. A downside to inertial navigation systems, he said, was that they lose accuracy over time and distance.
Goward said another future PNT technology is pseudolites, which are multiple, geographically-dispersed, terrestrial transmitters providing passive, or pseudo, ranging signals to help a vehicle accurately calculate its position. Passive ranging uses the transmissions of synchronized signals that are encoded with a means of determining station location and its time of transmission, allowing users to calculate total travel time, and range, by measuring the time of arrival.
A third possible PNT technology, Goward said, was called Enhanced Loran (eLoran), which broadcasts GPS time, but on a higher power and much lower frequency. Since it is of lower frequency, Goward said, eLoran is able to be brought indoors, underground and underwater. It is also difficult to disrupt, Goward added.